A wristwatch or other portable electronic clock containing a solar cell module or other power generation device and operating without having to replace batteries has been realized. A power generation device such as a solar cell module has the problem that the density of useable energy inside a room is low except when under direct sunlight and further that continuous energy cannot be obtained. For this reason, a watch has a power save function whereby it automatically enters a power conserving state when detecting that light is not striking the solar cell module and power has not been generated for a certain time. Further, as shown in PLT 1, even an electronic watch with an electromagnetic induction generator which captures movement of a user's arm etc. to convert kinetic energy to electrical energy has a power save function where a power conserving mode is entered automatically so as to reduce the power consumption of a storage battery (secondary cell) when not carried.
An electronic watch entering such a power save mode ends up with the liquid crystal display extinguished and the hands stopped, so the user often mistakenly believes it has broken down. For this reason, users often ask “My watch has stopped. What should I do?” Manufacturers have prepared answers to this as a typical frequently asked question (FAQ). To prevent such misunderstandings, the method is known of making a power saving mark flash or using other display means to inform the user of the situation. However, not only does the cost increase by the amount of provision of the display means, but also it becomes necessary to secure space for the display of the display means. There was the problem that the size of the watch ended up becoming larger by that amount. Further, when the level of charge of the secondary battery became lower and became the power becomes insufficient, it became necessary to place the solar cell module in direct sunlight to charge it and confirm operation. This often led to failure to immediately judge breakdown. It became necessary to deal with such doubts on the part of the consumers as well.
On the other hand, in recent years, practical power generating devices utilizing electrostatic induction by electret materials have been developed as disclosed in PLTs 2 to 5. “Electrostatic induction” is the phenomenon of a charge of an opposite polarity from a charged object being induced by the charged object being brought into proximity with a conductor. A power generation device utilizing the phenomenon of electrostatic induction generates power by utilizing this phenomenon to take out a charge induced by relatively movement of two members in a structure comprised of a “film holding a charge” (below, referred to as a “charging film”) and a “counter electrode”.
FIG. 1 is an explanatory view for explaining the principle of power generation utilizing the phenomenon of electrostatic induction.
If taking as an example the case of using an electret material, an electret is a dielectric material given an electric charge and generates an electrostatic field semipermanently. In power generation by this electret, as seen in FIG. 1, the electret 3 and the counter electrode 2 are brought into proximity. The electrostatic field formed by the electret causes formation of an induced charge at the counter electrode. If changing the overlapping area of the electret and counter electrode (by vibration etc.), it is possible to take out alternating current at the outside electrical circuit 300. Power generation by an electret is relatively simple in terms of structure and utilizes electromagnetic induction, so gives a high output in the low frequency region and is therefore advantageous. In recent years, it has been focused on for so-called “energy harvesting”.
PLT 2 discloses a power generating device providing a radially cut electrode and electret film at a rotating plate rotating with respect to a main plate provided at the watch module, providing a radially cut electrode on the main plate, making the rotating plate rotate with respect to the main plate, and converting the energy of rotary motion to electrical energy. PLT 3 also discloses a power generating device similar to the power generating device of PLT 2 provided with an electrode or electret film at a side surface of a rotary member and a surface facing the same. Furthermore, PLTs 4 and 5 disclose electrostatic induction type power generating devices using hairsprings (one type of spiral spring, watchmaker's term) to make the electret film and electrode move back and forth and periodically rotate. When applying these power generating devices to portable electronic watches, in the same way as the solar cell modules and other electronic watches or clocks explained above, the above problem arises that the user will frequently end up misconstruing the power save mode as a breakdown.
The prior arts of PLTs 2 to 5 all disclose a rotating disk provided with an electret film or electrode in a case or deep beyond a main plate. Rotational operation of the electret cannot be visually confirmed from the outside. For this reason, in the power save mode or when the charge of the secondary battery becomes low and therefore power becomes insufficient, the user could not immediately confirm the state of power generation.